The present invention relates to cellular communication systems, and more particularly to access response signaling in a cellular communication system.
Cellular communication systems typically comprise a land-based network that provides wireless coverage to mobile terminals that can continue to receive service while moving around within the network's coverage area. The term “cellular” derives from the fact that the entire coverage area is divided up into so-called “cells”, each of which is typically served by a particular radio transceiver station (or equivalent) associated with the land-based network. Such transceiver stations are often referred to as “base stations”. As the mobile device moves from one cell to another, the network hands over responsibility for serving the mobile device from the presently-serving cell to the “new” cell. In this way, the user of the mobile device experiences continuity of service without having to reestablish a connection to the network. FIG. 1 illustrates a cellular communication system providing a system coverage area 101 by means of a plurality of cells 103.
The radio frequency spectrum that is utilized to provide mobile communication services is a limited resource that must be shared in some way among all of the users in a system. Therefore, a number of strategies have been developed to prevent one mobile device's use (both transmitting and receiving) of radio spectrum from interfering with that of another, as well as to prevent one cell's communications from interfering with those of another. Some strategies, such as Frequency Division Multiple Access (FDMA) involve allocating certain frequencies to one user to the exclusion of others. Other strategies, such as Time Division Multiple Access (TDMA) involve allowing multiple users to share one or more frequencies, with each user being granted exclusive use of the frequencies only at certain times that are unique to that user. FDMA and TDMA strategies are not mutually exclusive of one another, and many systems employ both strategies together, one example being the Global System for Mobile communication (GSM).
As designers strive to develop systems with higher and higher capabilities (e.g., higher communication speeds, resistance to interference, higher system capacity, etc.), different technical features are incorporated, including different means for sharing radio frequency resources. To take one of a number of possible examples, the Evolved-Universal Terrestrial Radio Access Network (E-UTRAN) Long Term Evolution (LTE) technology, as defined by 3GPP TR 36.201, “Evolved Universal Terrestrial Radio Access (E-UTRA); Long Term Evolution (LTE) physical layer; General description” will be able to operate over a very wide span of operating bandwidths and also carrier frequencies. Furthermore, E-UTRAN systems will be capable of operating within a large range of distances, from microcells (i.e., cells served by low power base stations that cover a limited area, such as a shopping center or other building accessible to the public) up to macrocells having a range that extends up to 100 km. In order to handle the different radio conditions that may occur in the different applications, Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink (i.e., the communications link from the base station to the User Equipment—“UE”) because it is a radio access technology that can adapt very well to different propagation conditions. In OFDMA, the available data stream is portioned out into a number of narrowband subcarriers that are transmitted in parallel. Because each subcarrier is narrowband it only experiences flat-fading. This makes it very easy to demodulate each subcarrier at the receiver.
The use of machine type communication (MTC) over cellular communication systems, such as LTE, is increasingly gaining attention as operators are planning for replacement of older communication systems, like GSM, by newer (e.g., LTE) networks. MTC devices, such as connected sensors, alarms, remote control devices and the like, are common in GSM networks where they co-exist with more conventional UEs (e.g., mobile phones). MTC devices typically need to communicate only small amounts of data, and are therefore generally characterized by a modest bit rate and sparse communication activity. The number of MTC devices is expected to increase dramatically during the next few years, with predictions indicating that in only a few years, there will be hundreds of billions of such devices connected to cellular systems like LTE.
An important requirement of MTC devices is that they should have low cost as well as low power consumption. A device's power consumption can be a function of a number of cellular system parameters. One example that typically drives power consumption is the amount of time the device needs to monitor a control channel for information, such as the time the device needs to monitor (decode) a control channel signal to ascertain whether it includes an access request signal. A common example of a response signal is the Random Access Response (RAR) signal, which is a network node's acknowledgment of a Random Access signal transmitted by the device on a Random Access Channel (RACH) and received by the network node. In mobile broadband scenarios (e.g., the scenario the LTE system is primarily built on) it is important that the RAR be transmitted quickly after occurrence of the RACH in order to reduce latency. However, since a device's utilization of the RACH is an event that cannot be predicted by the network node, responding quickly to a random access burst typically means interrupting the scheduling of another user's data transmission. This in turn reduces the capacity/spectral efficiency of the system. Therefore, in order to overcome this problem to some extent, the device needs to be allocated a time window, longer than the actual RAR, during which the RAR can be transmitted.
FIG. 2 is a signal timing diagram of a conventional Random Access procedure such as is used in a conventional LTE (or comparable) system. The air interface is divided up into sequentially occurring sub-frames, of which the sub-frame 201 is but one example. It is assumed in this illustration that the mobile device has already synchronized itself to the network node. In FIG. 2, uplink (UL—from device to network node) and downlink (DL—from network node to device) timelines are shown separate and aligned with one another.
Not shown in FIG. 2 is earlier signaling that has been exchanged between the device and the network node by which the network node has provided the device with information about what signature to use when it is ready to make contact with the network. In this illustration, the device transmits a random access signal on a RACH channel 203, indicating that it intends to get in contact with the network node for the purpose of, for example, transmitting data information. The RACH is typically allocated a certain frequency bandwidth in one or more sub-frames. (This example assumes that the RACH 203 fits within a single sub-frame.) Typically, the RACH is of shorter duration than an entire sub-frame in order to cope with the fact that, at the time of the device's initial contact with the network node, the travel distance (and hence path delay) of the radio signal between the network node and the device is unknown.
The device turns on its transmitter (device activation step 205) and transmits the specific RACH signature (previously provided by the network node) that identifies the terminal to the network node. The network node detects the RACH signal some number of sub-frames later (step 207) and in response, transmits an RAR signal to the device (step 209), indicating procedures for further communication.
As mentioned earlier, in order to avoid interrupting the scheduling of another user's data transmission and thereby trying to optimize usage of the radio frequency spectrum and of the scheduler resources, a RACH response window 211 is defined. The network node's RAR signal can be transmitted to the device in any one of the sub-frames spanned by the RACH window 211 The RAR window 211, signaled (or broadcasted) from the network node is typically 5-10 sub-frames in duration, and hence the device needs to have the receiver (RX) on during the entire RAR window for monitoring of the RAR (device activation step 213). Considering the desire to keep power consumption in an MTC device very low, the long receiver “on” time relative to the short RAR information that is to be captured (a ratio that is on the order of 5-10 to 1), it is apparent that this is not a good solution.
There is therefore a need for improved signaling methods and apparatuses in cellular systems such as, but not limited to, an LTE system that addresses one or more of the shortcomings described above.